interface Person {
  name: string;
  age: number;
}

type K1 = keyof Person; // "name" | "age"
type K2 = keyof Person[]; // "length" | "toString" | "pop" | "push" | "concat" | "join" 
type K3 = keyof { [x: string]: Person };  // string | number

// a 只能是两个值
let a: K1;
a = "name";
a = "age"

let b: K2;
b = "length";
b = "concat";

let c: K3;
c = "213";
c = 213;
